1. Field of the Invention
The invention relates to a heart therapy device. More specifically, the invention relates to an automatically implantable cardioverter/defibrillator (AICD) or a combined demand pacemaker/AICD.
2. Related Art
Automatic heart therapy devices are generally known and of daily medical usexe2x80x94particularly and for a longer period of time as implantable heart pacemakers for the treatment of bradycardiac and/or tachycardiac arrythmiae, but also increasingly as automatic defibrillators or cardioverters, combined pacemakers/defibrillators or as implantable drug dosing pumps, and the like.
Generic devices are also particularly known, which are equipped with one or more sensor(s) for sensing the cardiac rhythm in the patient""s body, associated signal processing and editing means, and an evaluation and control unit. The latter calculates in each case an actual parameter or set of parameters in accordance with an algorithm stored in the device, depending on the value or the values of the heart rate from the amount of programmed operational parameters or therapy values. Therapy devices of said kind are known, which are configured for an automatic actuation orxe2x80x94a pre-programmedxe2x80x94switch-over from one operational mode into another depending upon the cardiac rhythm sensed in the patient""s body. Such devices include the known demand heart pacemakers or automatic defibrillators, and the combination devices developed in recent times.
As early as the development and clinical application of so-called demand pacemakers, it has been known to specifically control heart pacemakers such that spontaneous cardiac actions are sensed, and the value of the heart rate or of the time intervals between determined heart actions (e.g. the RR intervals between successive ventricle actions) are assessed and compared with a predetermined nominal value, and that the pacemaker outputs stimulation pulses exactly at the moment, when the measured value is not within the range defined by the nominal value.
More modem devices of this type are microprocessor-controlled and offer the possibility of an individual, tailor-made programming for specific clinical symptoms from a plurality of pre-installed operational modes, by means of which and of associated operational parameters (in the following also called therapy values, as far as these are therapeutically relevant), a predetermined therapy is realized.
Within this framework, essential further developments of the concept of the demand pacemaker to the universal demand heart rhythm correction device have taken place, some of them starting from an increasingly refined subdivision of ranges of the heart rate continuum or RR interval continuum and which, depending upon which of the plurality of predetermined ranges the actual measured value lies, one of a plurality of defined therapies in each case definitely allocated to one range, is realized. By means of such a device, classical demand pacemaker operation in the case of bradycardia can be realized as well as conventional therapies of various tachycardiae (cf. for example U.S. Pat. No. 4,181,133) or, if necessary, even a defibrillation shock therapy (cf. U.S. Pat. No. 4,300,567).
Because the sole allocation of the heart rate to a predetermined range does not always reliably allow for a determination of the adequate therapy, additional classification criteria have been increasingly tested in the developments of the past years, and have been taken into account with the control algorithms; Such an example can be found in U.S. Pat. No. 5,379,776 (including the therein quoted sources).
Devices of the aforementioned kind are programmed on the occasion of implantation according to the clinical symptoms and, in some cases, to the living conditions (e.g. the average physical activities) of the patient, whereby the algorithm to be applied is also fixed for determining the therapy or the therapy value(s) in dependence of t he value(s) sensed in the body. At the time of regular aftercare examinations taking place at determined intervals, the set of operational modes and parameters, as well asxe2x80x94in case the therapy device disposes of several stored algorithmsxe2x80x94the control algorithm to be applied can be modified by reprogramming.
U.S. Pat. No. 5,354,316 discloses a method and an apparatus for detecting and treating tachycardiae and fibrillations of a heart by means of specifically selected therapies of a combined pacemaker-cardioverter-defibrillator (PCD) apparatus of the generic kind. U.S. Pat. No 5,354,316, uses mutually overlapping range limits between tachycardia and fibrillation heart rate ranges. The assessment of the actually detected heart rhythmxe2x80x94in case of elevated heart ratesxe2x80x94in the area of the ventricular tachycardia or of the ventricular fibrillation, ensues in the overlapping ranges by means of additional assessment criteria. It is specifically examined how many of the preceding intervals between successive ventricle actions fall into the overlapping range.
U.S. Pat. No. 5,447,519 discloses a similar method, whereby as a differentiation criterion between a ventricular tachycardia and ventricular fibrillation, the variability of the heart signal morphology (ECG curve profile) is referred to.
Another generic device is described in the Applicant""s DE 196 09 362 C1. This heart therapy device likewise defines an overlapping region between two adjacent value ranges of a value measured in a patient""s body. For controlling a selected, predetermined therapy, this device generally takes an evaluation of the history of the measurable variable (specifically the range allocation of the respective preceding value) or even of the therapy control value itself as the basis, in case the value is within the overlapping range.
It is an object of the invention to provide an improved therapy device of the generic kind, having a relatively simple and cost-efficient structure and yet being able to provide for an utmost careful but simultaneously efficient therapy of tachycardiac arrythmiae bordering on fibrillationsxe2x80x94in particular in case of an atrial tachycardia.
The above and other objects of the invention can be solved by a heart therapy device. The heart therapy device includes a heart rate detection means for detecting a heart rate, an evaluation and control means coupled to the output of the heart rate detection means, the evaluation and control means having an output and being adaptable to evaluate a measured heart rate, including allocating the measured heart rate to one of a first, second and third heart rate value ranges, and output a therapy control signal in accordance with said allocation. The evaluation and control means includes a three-area memory for storing the first, second, and third heart rate value ranges, the heart rate value ranges ascending incrementally in value; a heart rate discriminator coupled to the heart rate detection means and the three-area memory for allocating a measured heart rate to the first, second or third heart rate value range, and for outputting a corresponding first, second or third discriminator output signal; a stability evaluation means for outputting a stability output signal and evaluating heart rate stability over a predetermined period of time in response to the output of the second heart rate discriminator output signal; and a logical processing unit for processing the second discriminator output signal and the stability output signal and for generating a second or third therapy control signal, while the first and third discriminator output signal are directly outputted as first and third therapy control signal, respectively. The heart therapy device also includes a therapy means coupled to the output of said evaluation and control means for realizing at least one therapy having a predetermined therapy parameter in response to the therapy control signal, in accordance with the measured heart rate lying within one of the predetermined value ranges, the therapy means being adaptable to output at least two different, uniquely pre-defined therapies in response to said second and third therapy control signal.
The invention embraces the essential idea of effecting an evaluation of high heart rates for deriving an adequate therapy by classifying the corresponding (high frequency) heart rate ranges into three adjacent ranges with fixed range limits. The invention thereby turns away from previous solutions according to the above-mentioned printed publications, in which the overlappings were in particular defined without rigidly predetermined limits. Moreover, the invention embraces the idea of using in the mid-range of the three value ranges apart from the heart rate itself, the result of a statistical evaluation of samexe2x80x94and namely specifically its stability in a preceding predetermined period of timexe2x80x94for deriving an appropriate therapy. By means of this approach, a simplification of the hardware and software configuration of the heart therapy device and an increase of the predictability and reliability of the therapy is achieved.
According to the an exemplary embodiment of the invention, it is provided that upon detection of a heart rate within the first value range (tachycardia zone 1), a first, uniquely pre-defined therapy control signal, and upon detection of a heart rate within the third value range (fibrillation zone), a third, likewise uniquely pre-defined therapy control signal is outputted. If, however, a heart rate is detected within the second value range (tachycardia zone 2), one of two different therapy control signals is outputted depending upon the stability of the heart rate. The stability criterion serves for distinguishing between xe2x80x9crapidxe2x80x9d tachycardia (xe2x80x9cflutterxe2x80x9d) and fibrillations of axe2x80x94stillxe2x80x94relatively low frequency. In particular, it is provided to output a second, specific therapy control signal in case of high stability for initiating a correction measure suitable for rapid tachycardia, whereas in the case of low stability, the third therapy control signal is outputted just as with a heart rate lying within the third value range.
This third therapy control signal, as a rule, will cause a shock therapy to be initiated, hence will trigger a cardioverter stage or defibrillator stage of the heart therapy device. As compared thereto, under certain circumstances, stimulation is not yet actually initiated at all by the first therapy control signal, rather, if necessary, the therapy device is first set into an operational state. However, the output of pacemaker pulse sequences to the heart may already be controlled by the first therapy signalxe2x80x94for example for an early control of the so-called re-entry tachycardia. By means of the second therapy control signal, a therapy is controlled, which lies in between the therapies with respect to its xe2x80x9caggressivenessxe2x80x9d, and which is used in tachycardia zone 1, for one, and in the fibrillation zone, for another. On the one hand, pacemaker pulse sequences can thereby be still concerned, e.g. the high frequency burst or ramp sequences (known per se), but also, on the other hand, a shock therapy (cardioversion) with one or more shock pulse(s) of relatively low energy. (The concept of low-energy cardioversion is also known per se and therefore does not require any further explanation here.)
The function of the therapy device outlined above hence resides in the finding thatxe2x80x94with particular respect to atrial rhythmxe2x80x94there is a rate range of high-frequency tachycardia between the range of not life-endangering (xe2x80x9cphysiologicalxe2x80x9d) tachycardia and the range of life-endangering fibrillations, in which the stability criterion is referred to for distinguishing between tachycardia and fibrillations and for correspondingly differentiating the therapy. A stable atrial rhythm is treated as atrial flutter by a pacemaker stage of the therapy device, thereby allowing avoidance of cardioversion shocks, which are unnecessary for this stage and painful to the patient. Comparable heart rates with low stability, however, are a sign of menacing atrial fibrillation, and require activation of a cardiaversion or defibrillator stage.
It can already be seen from the statements made above that in an exemplary embodiment of the device, the heart rate detection means for detecting the atrial rate is connected with the patient""s atriumxe2x80x94and namely in the practically most significant realization as an implantable device, as a rule via a detection electrode placed within the atrium and via a thereto connected measurement signal line. For special cases, other realizations are likewise possible, e.g. as an extracorporal device in clinical application and having extra-atrially arranged detection means. The use of the proposed device is further possible in high-frequency atrium arrythmiaexe2x80x94ventricular tachycardia/ventricular fibrillations.
One component of the proposed heart therapy device for realizing said therapy concept, is a three-area memory for storing the first, second and third value range of reference heart rates covering the total frequency range from relatively low-frequency, physiological tachycardia up to extremely high-frequency fibrillations. The limits between these ranges are fixedly determined for the ongoing operationxe2x80x94if necessary, however, adjustable by programmingxe2x80x94and namely according to the specific indication adapted to the specific conditions of the respective patient. Said three-area memory co-operates with a multistage heart rate discriminator, which effects the allocation of a measured heart rate to one of the value ranges, and delivers an output signal characterizing the allocation result. Said output signal in particular is at the same time the first or third therapy control signal, respectively, as far as the actually detected heart rate lies within the first or third value range.
Moreover, a stability evaluation means for evaluating heart rate stability over a predetermined period of time (or a predetermined number of preceding heart actions) co-operates with said heart rate discriminator. Said stability evaluation means is activated insofar as a corresponding output signal of the heart rate discriminator indicates a value measured in the second value range. The stability evaluation means delivers an output signal characterizing the stability of the heart rate, which output signal is processed in a logical processing unit together with the discriminator output signal, so as to obtain the heart rate value as well as the therapy control signal reflecting the stability thereof. Hence, by means of the stability evaluation unit and the logical processing unit, a so to speak xe2x80x9ctherapy commutatorxe2x80x9d between a less aggressive and a more aggressive heart rhythm correction measure between atrium (or ventricle) flutter, for one, and atrium (or ventricle) fibrillation, for another, is realized.
The stability evaluation means comprises the functional components necessary for the statistical evaluation of the heart rate values detected within a determined period of time, and namely in particular a heart rate measured value memory, a thereto connected statistical processing unit for determining a stability value, and a thereto connected stability discriminator stage, in which the actual stability value obtained with the statistical evaluation is compared with the predetermined (programmed) threshold value.
The therapy means of the device comprises at least one defibrillator, however, in the preferred embodiment, it further comprises a pacemaker means, and in particular also a therapy memory, by which, in response to the therapy control signals, in each case a predetermined therapy by the defibrillator or, optionally, the pacemaker, is activated. The therapy memory is connected to the output of the evaluation and control means in such a manner that the therapy control signals address in each case a predetermined memory area, where therapy data of a specific therapy is stored. Addressing of the first and third memory area ensues directly from the heart rate discriminator stage, whereas for addressing the second memory area the output signal of the stability evaluation means is necessarily used, and it is used in addition for addressing the third memory area. Hence, corresponding AND gates are provided here.
With respect to the specific therapies and hence also the therapy data stored in the therapy memory, there exist wide variation possibilities within the framework of the present invention, depending on the specific use of the device for a therapy of atrium or ventricle arrhythmiae, and on the configuration simply as a cardioverter/defibrillator or as a combined demand pacemaker/defibrillator. Thus, with a simple defibrillator, the first therapy control signal may cause the initiation of the operational state, whereas the second and third therapy control signal trigger shock therapies with different pulse energies. With a combined pacemaker/defibrillator, however, the first and second therapy control signals will preferably trigger a first and second stimulation pulse sequence, whereas the third therapy control signal triggers a shock pulse. For the remainder, reference is made to the basic explanations made above.
A reliable function of the proposed device requires permanent or periodical sampling of the heart rate, whereby at least with detection of a heart rate lying within the total value range of increased heart rates, the mentioned value range discrimination and, if necessary, stability evaluation, are triggered. For this purpose, a suitable controller is present in the evaluation and control means.